I'd create a virus that 'has no vaccine', administer said 'impossible' vaccine to myself and a few loved ones. Also, leave the vaccine in some water wells, ect to let some of Earth's population live. This virus would cause a slow, painful death.
So, eventually I'd pop up with my vaccine, and offer it to people, claiming to be Jesus or something, and the only way they can be treated is if they follow me. I'd build a large Communistic society with a new religion where I am Savior, and Satan takes the form of disease- because disease kills us, our loved ones, and makes us act desperately. Every few years, I'd bring up a new virus to slim my population down and to bring back the faith in my people.
Some one would catch on eventually, but I'd have a good run until then.
I'd also create a super virus, which has no cure,
It is a rabies-ebola-flu-AIDS-herpes-syphilis hybrid virus. Only 22% mammals are immune to it. If you get that super virus
At first you will get flu like symptoms
Then you starting loosing higher brain functions
You fall into a coma and die
The body cells and nervous systems restart and the body is reanimated. It can only be neutralized by headshots or decapitation...
Answer on How can I achieve world domination?
You'd also need to grow a small hitler-stache.